The present invention relates generally to devices for establishing prerotation of aircraft wheels during landing operations. More particularly, the invention is directed to extendable and retractable devices for use in prerotation of aircraft wheels.
In the years since aviation has become widespread both as a leisure activity and as a commercial activity, it has been known that during landing operations aircraft tires experience rather severe duty cycles. For example, it will be readily observed that as an aircraft wheel first touches the ground during a landing, it is in a non-rotating condition relative to the aircraft. Yet the aircraft itself can be moving relative to the ground at a speed of 50-250 miles per hour, depending on the nature and size of the aircraft. More particularly, the landing speed of an aircraft is a function of its weight to lift ratio. For example, a small fighter may have a higher landing speed than a large cargo plane. As the aircraft wheel touches down, it must begin to rotate at an angular velocity such that its tangential speed matches the speed of the aircraft relative to the ground.
During the time interval required for the wheel to attain that angular velocity, the tire skids on the runway pavement. That skidding leads to loss of material from the tire itself as well as to generation of considerable heat generated by friction between the tire and the runway. The result of that skidding is extremely short duty life for aircraft tires. That short life is even further abbreviated as the size of the aircraft increases. And with larger aircraft, the tires themselves become even more expensive than with smaller aircraft.
Steps which will reduce the severity of the aircraft tire duty cycle will increase the useful life of the aircraft tire. Concomitantly, there is a substantial economic benefit since larger, more expensive aircraft tires can be used for longer periods of time without replacement. Accordingly, apparatus which can accomplish this goal is economically desirable.
In addition, skidding of aircraft tires on the surface of runways causes a layer of rubber to accumulate on runway surfaces. This layer must be removed periodically, since accumulations can affect traction on landing, can cause hydroplaning under wet conditions, and can damage jet engines if pieces of the layer become dislodged and are ingested by jet engines on takeoff. The need to remove rubber layers adds to the expense of airport operations and aircraft landing fees.
In the past, various proposals have been advanced which attempt to deal with the landing stress imposed on aircraft tires. For example, it has been proposed to use cup-shaped devices attached to both sides of an aircraft wheel to get aircraft wheels rotating before the aircraft itself touches down. See, U.S. Pat. No. 2,339,241, issued to Davis on Jan. 18, 1944 and U.S. Pat. No. 4,040,582, issued to Krauss on Aug. 9, 1977. Such arrangements were not satisfactory, however, since the permanent arrangement of the devices on the aircraft wheel caused increased drag on the aircraft leading to less economical operation, and under some circumstances, reduced speed and greater required distance for take-off.
To reduce the drag exerted by the prerotation device, it has been proposed to make the devices retractable during flight of the aircraft. One such concept used a Bowden wire cable that was actuated by the pilot to control the position of the retractable devices. See, U.S. Pat. No. 2,397,319, issued to Johnson, Jr., on Mar. 26, 1946.
Effective prerotation is accomplished by imparting torque to the aircraft wheel. That torque is enhanced when the resistance of a device moving forwardly relative to the aircraft is less than the resistance of the device moving rearwardly relative to the aircraft. Devices which move in and out in response to wind pressure in order to change their drag characteristics have been proposed for permanent attachment to an aircraft wheel, as have devices made of flexible materials. See, U.S. Pat. No. 2,435,459, issued to Oden on Feb. 3, 1948 and U.S. Pat. No. 2,941,758, issued June 21, 1960 to R. Cordoba. Such devices, however, cause unbalanced dynamic forces to operate on the wheel structure due to the changes in position. Since unbalanced dynamic forces lead to vibrations, devices which cause them are undesirable.
It has also been recognized that wheel prerotation devices are more effective as they extend beyond close proximity to the aircraft wheel. Thus, devices have been suggested in which springs as well as fluid pressure, project cup-shaped members outwardly from the wheel. See, U.S. Pat. No. 2,666,604, issued to Davis on Jan. 19, 1954.
In a somewhat similar vein, it has been recognized that wheel prerotation devices are more effective when operating at large diameters. As a result it has been proposed to move the radial location of devices to near the periphery of the tire to enhance rotation efficiency.
Generally speaking, the prerotation devices heretofore available provide torque to rotate the aircraft wheel by virtue of the differences in drag available at the top and the bottom of the rotation cycle. When the devices are in line with one another, they do not contribute to the wheel rotation torque. Thus, those devices do not exert a driving force on the wheel through an angle greater then 180 degrees.
Furthermore, the known devices do not provide a mechanism for controlling deployment of the prerotation element so that the position of all elements is uniform. And, it is important to provide the ability to regulate the amount of deployment of wheel rotation devices so that the aircraft wheel will not exceed the angular velocity necessary to match ground speed of the aircraft.
According to the present invention, wheel prerotation vanes are provided which have an airfoil shape in cross section. These vanes are oriented such that the chord line of the airfoil section is aligned to be radial or at a small angle relative to a radius of the wheel. As a result the natural concavity of the airfoil section provides a greater drag when the vane is located at the bottom of the wheel than when the vane is located at the top of the wheel. But, when the airfoil sections are aligned, each creates a lift force. The lift force of the forward vane is directed vertically downwardly whereas the lift force of the rearward vane is directed vertically upwardly. Thus, a significant contribution to the wheel rotation torque is accomplished throughout the various rotational positions of the vanes around the wheel.
The vanes are preferably inclined so as to be swept backward away from the direction of wheel rotation. This arrangement provides a reduction in the aerodynamic drag acting on vane when the vane passes around the top portion of the wheel since the backward sweep reduces the drag coefficient for the vane. Moreover, the backward sweep increases the drag coefficient for the vane as it passes around the bottom portion of the wheel since the vane presents more of a cup like surface to the passing airstream. This combination of aerodynamic effects augments the rotational effect of the vanes of the present invention in comparison to vanes which are not backwardly swept.
When it is desired that the vanes be deployable, an hydraulic system may be provided to control deployment as well as to control retraction of the vanes. This hydraulic system is operable to position the vanes at continuous increments between a fully retracted position and a fully deployed position.
So that the deployable vanes will be uniformly positioned, means may be provided to simultaneously deploy and retract the vanes. One suitable device is a unison ring which engages arcuate gears on each of the vanes.